QC 

4-37 


UC-NRLF 


B  ^ 


THE  ABSORPTION  OF  LIGHT 
BY  INORGANIC  SALTS 


DISSERTATION 

Presented   In   Partial   Fulfillment  of  the   Requirements  for 

the  Degree  of  Doctor  of  Philosophy  In  the  Graduate 

School  of  the  Ohio  State  University. 


BY 


ENOCH  FRANKLIN  GEORGE 


The  Ohio  State  University 
1920 


EXCHANGE 


THE  ABSORPTION  OF  LIGHT 
BY  INORGANIC  SALTS 


DISSERTATION 

Presented  In  Partial   Fulfillment  of  the   Requirements  for 

the  Degree  of  Doctor  of  Philosophy  in  the  Graduate 

School  of  the  Ohio  State  University. 


BY  •*., 


ENOCH  FRANKLIN  GEORGE 


The  Ohio  State  University 
1920 


•  ••••  • 


&s^-+£^ 


INTRODUCTION. 

The  mechanism  of  the  absorption  of  light  is  one  of  the  least 
understood  of  physical  phenomena.  The  literature  upon  the  subject 
is  voluminous,  but  not  always  illuminating.  The  matter  has  been 
dealt  with  by  theoretical  and  experimental  physicists  and  chemists. 
On  the  experimental  side  there  has,  in  general,  been  no  attempt  to 
distinguish  btween  absorption,  scattering,  and  reflection.  In  dealing 
with  solutions,  especially,  the  difference  between  the  intensity  of  the 
incident  light  and  that  transmitted  has  been  taken  as  a  measure  of 
the  absorption,  although  the  decrease  in  intensity  is  due  to  all  three 
factors.  This  seems  to  be  unavoidable.  Certainly  it  is  not  easy  to 
differentiate  between  them  experimentally.  But  in  any  theoretical 
investigation  or  discussion,  some  attempt  should  be  made  to  distinguish 
between  them. 

When  an  electromagnetic  wave,  as  a  light  wave,  is  incident  upon 
a  material  dielectric,  there  is  set  up  in  the  medium,  in  addition  to  the 
displacement  current  in  the  ether,  a  polarization  current  due  to  the 
periodic  displacement  of  the  electrons  in  the  atoms  or  molecules  of 
the  medium,  and  the  magnitude  of  this  current  will  be  proportional 
to  the  time  rate  of  change  of  the  electric  intensity.  If  the  displacement 
of  the  electrons  is  resisted  by  some  force  similar  to  that  of  friction, 
energy  will  be  absorbed  from  the  waves  and  converted  into  heat,  as 
when  a  condenser  is  heated  by  a  rapidly  alternating  current.  If  in 
addition  to  the  bound  electrons  of  a  dielectric  there  are  also  free 
electrons,  as  in  a  metal,  there  will  be  conduction  currents  propor- 
tional to  the  electric  intensity.  Thus  again  we  will  have  heat  gen- 
erated at  the  expense  of  the  radiant  energy. 

The  chief  difficulty  with  the  foregoing  explanation  of  light 
absorption  is,  that  it  does  not  fit  in  with  observed  facts.  The  reason 
is  obvious.  We  have  neglected  the  inertia  of  the  electrons.  True, 
the  inertia  of  an  electron,  when  compared  with  that  of  familiar  things, 
is  small.  But  when  acted  upon  by  a  force  whose  direction  changes 
through  180°  a  million  billion  times  per  second,  the  inertia  becomes 
very  considerable. 

The  reduction  in  the  intensity  of  light  in  passing  through  material 
media  is  generally  thought  of  as  a  reasonance  effect.  Resonance  is 
a  phenomenon  encountered  in  every  branch  of  physics.  A  massive 
pendulum  can  be  set  into  violent  motion  by  a  tiny  force  if  applied 


•:  •.;<      /./  •  •/::..: 

/**••*«•*•*»*         4   »    k*«  V  *    •   *•» 


periodically  at  the  proper  time— that  is,  if  the  frequency  of  the 
applied  force  is  made  exactly  equal  to  that  of  the  natural  frequency 
of  oscillation  of  the  pendulum  This  process  of  adjusting  one  fre- 
quency to  that  of  another  is  known  as  "tuning".  If  one  tuning  fork 
is  set  vibrating,  another  of  exactly  the  same  frequency  will  take  up 
the  vibration  and  thus  absorb  some  of  the  energy  of  the  first  fork, 
while  a  third  fork  that  is  not  so  tuned  will  not  be  disturbed.  The 
wireless  operator  adjusts  the  capacity  and  inductance  of  his  sending 
apparatus  until  it  oscillates  with  a  certain  frequency,  which  is  the 
frequency  of  the  radio  waves  transmitted  through  the  ether.  At  the 
other  end  the  operator  must  adjust  the  capacity  and  inductance  of  his 
receiving  set  until  its  frequency  of  oscillation  is  exactly  equal  to  that 
of  the  oncoming  radio  waves;  that  is,  he  must  tune  his  receiving  set 
until  its  frequency  of  oscillation  is  equal  to  that  of  the  electromagnetic 
waves  propagated  through  space.  When  properly  tuned  it  responds  to 
the  signals  and  absorbs  energy  from  the  ethereal  waves.  In  light  we 
have  many  examples  of  this  effect.  The  characteristic  lines  of  metal- 
lic vapors  may  be  "reversed",  that  is,  the  bright  emission  lines  of  a 
glowing  vapor  are  reversed  and  become  dark  absorption  lines  when 
the  vapor  is  placed  in  the  path  of  a  more  intense  light  that  gives  a 
continuous  spectrum.  Certain  phenomena,  however,  are  not  easily 
explained  by  resonance. 

Vapors  of  sodium,  potassium,  iodine,  oxides  of  nitrogen,  etc., 
show  thousands  of  very  fine  absorption  lines,  whose  spectral  distri- 
bution follows  more  or  less  definite  laws,  as  observed  by  Deslandres 
and  others,  and  the  emission  spectrum  is,  broadly  speaking,  the  com- 
plement of  the  aborption  spectrum.  It  is  easy  to  assume  that  for 
each  line  there  is  a  set  of  (resonators  or  oscillators  whose  frequency 
is  identical  with  that  of  the  spectral  line,  and  that  they  remove  from 
the  incident  radiation,  the  frequency  corresponding  to  their  own.  But 
this  explanation  is  not  quite  sufficient.  Iodine  vapor  illuminated  with 
monochromatic  light  emits  a  series  of  isolated  bright  lines,  spaced  at 
nearly  equal  distances  throughout  the  spectrum.  Only  one  frequency 
can  be  absorbed,  since  monochromatic  light  is  used.  According  to 
Wood  the  absorbed  energy  is  then  re-distributed  among  other  fre- 
quencies by  some  mechanism  within  the  molecule.  Another  stumbling 
block  in  the  path  of  the  resonance  theory  is  found  in  the  behavior  of 
Roentgen  rays.  According  to  this  theory  any  substance  should  be 


opaque  to  a  radiation  whose  frequency  is  the  same  as  that  of  one 
of  its  characteristic  emission  lines.  It  is  well  known,  however,  that 
any  substance  is  unusually  transparent  to  X-rays  whose  frequency  is 
identical  with  that  of  one  of  its  own  characteristic  radiations.  Thus 
the  behavior  of  X-rays  is  diametrically  opposite  to  that  which  the 
phenomenon  of  resonance  would  lead  us  to  expect. 

Assuming  the  validity  of  the  resonance  theory,  and  certainly 
none  better  has  been  advanced,  just  how  does  it  account  for  the  reduc- 
tion in  the  intensity  of  light?  Presumably  the  vibrating  electron 
extracts  from  the  incident  radiation  the  energy  of  that  frequency 
corresponding  to  its  own,  and  re-radiates  it  in  all  directions.  But  this 
is  scattering,  and  not  absorption.  If  this  energy  is  ire-radiated  in  all 
directions  there  will  be  one  beam  in  the  direction  of  the  incident  beam, 
but,  according  to  Planck,  in  the  re-radiated  waves  there  will  be  a  lag 
in  phase  of  180  degrees  and  consequently  destructive  interference. 
There  is  also  a  beam  in  the  opposite  direction,  and  since  there  is  no 
other  wave  in  this  direction,  there  is  no  destructive  interference.  Con- 
sequently we  should  expect  to  find  a  beam  traveling  backward  from 
the  medium.  But  this  is  only  selective  reflection. 

Does  an  electron  absorb  energy  in  quanta?  Is  this  absorption 
accompanied  by  a  shift  to  an  orbit  of  greater  radius,  with  its  attendent 
increase  in  potential  energy?  Absorption,  when  strictly  defined, 
means  a  conversion  of  a  part  of  the  radiant  energy  into  heat  energy, 
v/ith  its  consequent  increase  in  temperature.  But  temperature  is  a 
function  of  molecular  velocity,  that  is,  of  the  kinetic  energy  of  the 
molecules.  How  dose  increase  in  electronic  energy,  that  is,  intra- 
atomic  energy,  increase  the  velocity  of  molecules?  How  dose  trans- 
ference of  energy  between  elctrons  and  molecules  take  place?  Ap- 
parently it  does  not  take  place  unless  it  occurs  during  molecular 
collision.  Just  what  the  process  of  transfer  is,  no  one  seems  to  know. 
According  to  Schuster  and  Nagaoki,  absorption  is  probably  due  to 
sudden  changes  in  vibration  caused  by  molecular  impacts. 

According  to  most  of  the  current  theories  of  atomic  structure 
the  atom  consists  of  a  number  of  electrons  equal  to  the  atomic  number 
of  the  atom,  revolving  in  elliptic  orbits  about  a  positive  nucleus.  The 
number  and  position  of  electronic  orbits  for  a  given  atom  under  given 
conditions  is  perfectly  definite,  but  the  number  of  orbits  may  be 
exceedingly  large  in  comparison  with  the  number  of  electrons.  That 
is,  an  electron  may  shift  from  one  orbit  to  another,  radiating  energy 


in  definite  quanta  if  the  shift  is  inward,  and  absorbing  energy  if  the 
shift  is  outward.  The  shift  of  an  electron  from  one  orbit  to  the  near- 
est orbit  on  either  side  is  characterized  by  the  absorption  or  emission 
of  one  quantum  of  energy. 

An  electron  in  motion  is,  by  definition,  an  electric  current.  A 
number  of  electrons  revolving  in  coplanar  orbits  about  a  common 
center  is  essentially  an  electromagnet.  Two  atoms  would  naturally 
attract  or  trepel  each  other  depending  upon  their  orientation.  If  two 
or  more  atoms  combine  there  will  certainly  be  a  rearrangement  of  the 
magneic  lines  of  force.  According  to  Humphreys,  the  electromagnetic 
"force  fields"  of  the  individual  atoms  "condense"  with  the  emission 
of  energy,  to  form  the  force  fields  of  the  resultant  molecule. 

What  effect  does  the  union  of  two  or  more  atoms  in  the  forma- 
tion of  a  molecule  have  upon  the  characteristic  frequencies  of  the 
atoms?  Perhaps  the  best  answer  has  been  given  by  Baly.  According 
to  him,  when  two  atoms  unite  to  form  a  molecule,  the  individual 
atoms  may  still  vibrate  with  their  characteristic  frequencies,  but  added 
to  these  are  frequencies  which  are  characteristic  of  the  molecules; 
these  frequencies  being  the  least  common  multiple  of  the  atomic  fre- 
quencies. In  a  more  complex  molecule  we  would  have  the  fre- 
quencies characteristic  of  the  individual  atoms,  frequencies  character- 
istic of  atomic  groups  within  the  molecule,  and  frequencies  char- 
acteristic of  the  molecule. 

The  width  of  an  absorption  band  is,  according  to  Baly,  due  to 
subsidiary  frequencies.  According  to  Bjerrum  (Zeitsch.  Electrochem. 
17,  731,  (1911);  ibid,  18,  101.  (1912);  Nernst,  Festschrift,  p.  90, 
(1912).)  the  subsidiary  frequencies  are  due  to  the  rotation  of  the 
molecules.  If  a  molecule  has  a  frequency  of  vibration  v,  and  a 
frequency  of  rotation  u,  then  it  will  exhibit  frequencies  v  plus  u  and 
v  minus  u  as  well  as  v.  Since  the  rotational  frequencies  of  different 
molecules  are  nturally  different,  we  should  expect  the  absorption 
bands  of  any  element  to  change  with  its  chemical  composition. 

If  an  atom  consists  of  a  system  of  coplaner  irings  of  electrons, 
each  of  definite  mass,  revolving  in  the  same  sense  about  a  positive 
nucleus,  then  the  atom,  due  to  its  configuration,  will  have  a  certain 
moment  of  inertia,  and  on  account  of  the  revolution  of  the  electrons, 
will  possess  a  certain  angular  momentum.  It  is  easy  to  see  that  the 
atom  will  function  as  a  gyroscope.  Rotation  of  the  atom,  in  the 
ordinary  sense,  will  not  occur.  Molecular  and  atomic  impacts  will,  in 


general,  be  followed  by  precessional  vibrations,  or  "wabbles",  as 
when  an  ordinary  gyroscope  is  struck  a  sharp  blow.  The  frequencies 
of  these  vibrations  will  be  a  function  of  the  moment  of  inertia  of 
the  atom,  which,  in  turn,  is  a  function  of  the  number  and  distribution 
of  the  electrons  in  their  orbits.  The  moment  of  inertia  of  a  neutral 
atom  will  be  different  from  that  of  an  ionized  atom,  and  the  moment 
of  inertia  of  a  free  atom  will  be  quite  different  from  that  of  the  same 
atom  in  chemical  combination,  because  the  configuration  of  the  elec- 
trons will  be  different. 

If  we  assume  the  validity  of  the  principle  of  equipartition  of 
energy,  then  from  a  study  of  the  experimental  values  of  the  molecular 
heats  of  gases,  we  must  conclude  that  diatomic  gas  molecules  possess 
only  five  degrees  of  freedom;  that  is,  one  degree  of  freedom  is  ap- 
parently missing.  There  are,  of  course,  three  degrees  of  freedom 
of  translation,  which  leaves  only  two  to  be  accounted  for.  Kruger 
has  interpreted  this  fact  as  evidence  in  favor  of  his  theory  of  pre- 
cessional vibrations,  which  corresponds  to  two  degrees  of  freedom. 
These  precessional  vibrations  are,  according  to  Kruger  (Annal.  der 
Physik.  IV,  50,  346;  ibid,  51,  450,  1916),  responsible  for  the 
subsidiary  frequencies. 

According  to  Baly,  the  subsidiary  frequencies  associated  with  a 
given  true  molecular  frequency  may  be  represented  by  the  expression 
M  ^nl  ±mA,  where  M  is  the  particular  molecular  frequency  concern- 
ed, I  is  one  or  the  other  of  the  intra-molecular  frequencies,  A  stands 
for  an  atomic  frequency,  while  m  and  n  stand  for  the  numbers 
0,  1,2,  3,  4,  etc.  The  center  of  the  band  will  be  found  by  making 
m  =  0  and  n  =  0.  This  will  result  in  the  entire  group  of  subsidiary 
frequencies  which  are  associated  with  a  given  molecular  frequency, 
being  divided  into  sub-groups.  Thus  for  a  gas  we  should  expect  the 
absorption  curve  to  consist  of  a  series  of  isolated  peaks  arranged  in 
groups,  the  whole  being  more  or  less  symmetrically  placed  with 
respect  to  the  molecular  frequency  M.  The  various  peaks  will  de- 
crease in  height  as  they  irecede  from  the  center  of  the  band.  If  the 
envelope  of  the  peaks  is  drawn,  we  should  have  a  smooth  curve  fall- 
ing off  on  either  side  of  the  frequency  M.  On  account  of  the  crowding 
of  the  molecules  in  a  liquid,  the  vibrations  and  oscillations  are  more 
or  less  forced,  so  that  absorption  does  not  consist  of  sharp  lines,  but 
is  more  or  less  continuous.  What  we  get,  then,  is  a  band  whose 
curve  is  approximately  that  of  the  envelope  mentioned  above. 


It  is  thus  evident  that  chemical  action — that  is,  regrouping  of 
atoms — may  exert  a  very  great  influence  upon  absorption  of  light  if 
the  atoms,  either  individually  or  in  combination,  have  characteristic 
frequencies  which  fall  within  the  visible  spectrum.  As  an  example 
of  this  effect,  it  is  well  known  that  the  introduction  of  certain  colorless 
compounds,  known  as  chromophores,  result  in  colored  compounds. 

One  quantum  of  energy  absorbed  at  the  higher  frequency  might 
be  re-radiated  at  the  same  frequency  (resonance  radiation),  or  as  a 
whole  number  of  quanta  at  the  lower  frequencies.  It  seems  reason- 
able to  suppose  that  the  reverse  process  might  also  take  place;  that 
is,  several  quanta  absorbed  at  the  lower  frequencies  might  combine 
according  to  the  principle  of  the  least  common  multiple  and  be  given 
out  again  as  a  single  quantum  at  a  higher  frequency.  The  true 
molecular  frequency  is  the  convergence  frequency  of  the  atomic 
frequency  series.  By  means  of  this  principle  of  the  combination  and 
resolution  of  frequencies,  Baly  has  apparently  offered  a  simple  ex- 
planation of  fluorescence  and  phosphorescence.  It  is  apparently 
similair  to  the  analysis  of  a  wave  by  means  of  a  Fourier's  series. 

According  to  Lambert's  law,  which  holds  strictly  within  the  limit 
of  experimental  error  for  monochromatic  light  of  any  wave-length, 
layers  of  any  homogeneous  substance  of  equal  thickness  absorb  the 
same  fraction  of  the  light  incident  upon  them.  In  other  words,  there 
is  an  exponential  relation  between  the  intensity  of  the  transmitted 
light  and  the  thickness  of  the  absorbing  medium.  According  to  Beer's 
law  the  absorption  of  light  by  a  fluid  of  definite  thickness  is  directly 
proportional  to  the  concentration,  or,  stated  another  way,  the  absorp- 
tion is  constant  so  long  as  the  product  of  the  concentration  and  thick- 
ness is  kept  constant.  Beer's  law  holds  approximately  in  a  large  num- 
ber of  cases  and  fails  completely  in  perhaps  a  larger  number.  It  is 
well  known  that  the  position,  intensity  and  width  of  the  absorption 
bands  of  many  solutions  vary  greatly  with  the  concentration  as  well 
as  with  the  nature  of  the  solvent,  even  when  the  product  of  con- 
centration and  thickness  is  kept  constant.  In  other  words,  the  so- 
called  extinction  coefficient  is  not  independent  of  the  concentration. 
And  further,  if  to  a  solution  of  a  colored  salt,  say  ferric  chloride,  a 
solution  of  a  colorless  salt,  as  ammonium  chloride,  is  added,  a  pro- 
found change  in  the  absorption  takes  place. 

Jones  has  explained  these  changes  in  terms  of  his  "Solvate 
Theory" ;  or,  rather,  he  has  interpreted  these  phenomena  as  one  link 


in  the  long  chain  of  evidence  in  favor  of  his  solvate  theory.  Accord- 
ing to  this  theory  when  a  salt  goes  into  solution,  both  the  ions  and 
the  molecules  of  the  salt  become  "solvated",  that  is,  form  loose  ag- 
gregates with  the  solvent.  When  copper  chloride,  for  instance,  is 
dissolved  in  water,  both  the  copper  ions  and  the  molecules  of  copper 
chloride  are  attached  to  large  numbers  of  molecules  of  water.  The 
frequency  of  vibration  of  the  resonators  of  these  aggregates  is  natur- 
ally different  from  that  of  the  free  molecule  of  the  salt  or  of  the 
copper  ion.  The  number  and  size  of  these  aggregates  depend  upon 
the  concentration,  consequently  the  extinction  coefficient  is  not  inde- 
pendent of  the  concentration.  According  to  this  conception,  if  a 
colorless  salt,  as  calcium  chloride,  is  now  added,  it  proceeds  to  form 
aggregates  with  the  molecules  of  the  water,  and,  in  general,  will  rob 
the  copper  ions  and  molecules  of  some  of  their  water  of  solvation. 
The  resultant  effect,  of  course,  will  be  a  change  in  absorption. 

Since  our  knowledge  of  the  many  variables,  both  physical  and 
chemical,  which  enter  into  this  problem  is  so  slight,  and  since  the 
investigations  of  Jones  and  his  co-workers  were  so  extensive,  the 
writer  has  no  desire  to  seriously  question  the  validity  of  the  solvate 
theory  as  applied  to  absorption  phenomena,  but  should  like  to  venture 
an  alternative  explanation  which  seems  more  simple  and  perhaps 
clearer.  It  is  well-known  that  the  degree  of  dissociation  of  an  elec- 
trolyte is  a  function  of  the  concentration;  the  more  dilute  the  solu- 
tion, the  more  nearly  complete  is  the  dissociation.  We  might  well 
expect,  also,  that  the  absorption  of  light  by  a  salt  in  the  ionic  state 
is,  or  may  be,  quite  different  from  what  it  is  in  the  molecular  state; 
whether  the  reason  for  it  is  that  given  by  Baly,  by  Bjerrum,  by 
Kruger,  or  some  other.  If  we  cannot  accept  this  assumption,  we  can 
see  no  reason  for  accepting  Jones's  assumption  that  the  absorption 
is  influenced  by  the  state  of  aggregation.  On  the  assumption,  then, 
that  the  absorption  is  a  function  of  the  dissociation,  it  is  easy  to  see 
why  Beer's  law  does  not  hold. 

If  to  an  aqueous  solution  of  a  colored  salt,  as  copper  chloride, 
in  a  definite  state  of  dissociation,  we  add  a  certain  amount  of  sodium 
chloride,  keeping  the  concentration  of  the  copper  chloride,  in  gram 
molecules  per  liter,  constant,  we  should  naturally  expect  that  some 
of  the  copper  ions  would  be  driven  back  into  the  molecular  state,  be- 
cause of  the  increase  in  concentration  of  the  chlorine  ions.  This 
statement  may  be  justified  by  the  law  of  mass  action  of  Guldberg  and 


10 

Waage,  or  by  the  principle  of  van't  Hoff  that  molecules  and  ions  in 
solution  behave  as  gases  and  obey  all  the  gas  laws.  This  seems  to 
explain,  at  least  as  well  as  the  solvate  theory,  the  changes  in  absorp- 
tion which  take  place  when  a  colorless  salt  is  added  to  a  colored  one 
in  a  binary  solution. 

It  is  well  known  that  increase  in  temperature  is  generally  ac- 
companied by  marked  changes  in  the  absorption  of  light.  The  ab- 
sorption band  usually  broadens  and  deepens  and  sometimes  shifts  to 
the  right.  Jones  attributes  this  to  the  splitting  up  of  the  molecular 
aggregates  and  the  formation  of  simpler  complexes.  It  seems  that 
this  effect  could  be  readily  explained  by  changes  in  ionization  with 
changes  in  temperature.  Bohr,  in  order  to  explain  the  fact  that  many 
lines  of  the  Balmer  series  of  hydrogen  are  observed  in  the  spectra 
of  the  heavenly  bodies  which  are  not  found  in  the  laboratory,  ad- 
vanced the  hypothesis  that  the  electrons  do  not  occupy  the  outer 
orbits  except  when  the  pressure  is  vanishingly  small ;  in  other  words, 
the  pressure  exerted  by  neighboring  molecules  restrain  the  electrons 
from  occupying  the  outermost  orbits.  This  explanation  is  accepted 
as  rational  by  several  subsequent  writers.  Now  if  the  pressure  of  the 
other  molecules  exerts  an  inward  force  on  the  electronic  orbits,  then, 
since  action  and  reaction  are  equal  and  opposite,  it  is  reasonable  to 
suppose  that  the  electrons  exert  a  pressure  tending  to  keep  the  mole- 
cules, and  in  many  cases  the  atoms  of  the  molecules  apart.  If  the 
absorption  of  energy  by  the  electrons  is  accompanied  by  shifts  to 
orbits  of  greater  radii,  we  might  well  expect  that,  in  general,  the  ab- 
sorption of  energy  by  the  electrons  would  tend  to  increase  dissociation 
in  those  molecules  where  the  electrons  of  all  the  atoms  are  not  revolv- 
ing in  orbits  about  a  single  axis.  According  to  this  line  of  reasoning, 
increase  in  temperature  would  probably  be  accompanied  by  increased 
dissociation.  Very  often,  however,  other  forces  may  be  brought  into 
play  which  tend  to  accelerate  the  reverse  process. 

According  to  Jones,  however,  the  above  conclusions  are  wrong. 

In  discussing  this  phase  of  the  subject  he  says :  "Rise  in  tempera- 
ture not  only  does  not  increase  the  number  of  ions  present,  but,  as  is 
well  known,  diminishes  dissociation."  It  is  doubtful,  however,  if  this 
sweeping  statement  can  be  justified.  Walker  says,  for  instance: 
"Since  the  heat  of  dissociation  into  ions  is  sometimes  positive,  some- 
times negative,  a  rise  of  temperature  may  in  some  cases  be  accomp- 


11 

anied  by  increased  dissociation,  in  others  by  diminished  dissociation." 
This  seems  to  be  in  accordance  with  Le  Chatelier's  theorem. 

It  seems  quite  probable,  however,  that  some  of  the  changes 
which  take  place  are  far  too  complex  to  be  explained  either  by  the 
solvate  theory  of  Jones  or  by  the  changes  in  ionization.  There  are 
probably  secondary  reactions  which  play  an  important  role.  For 
instance,  Baly  (Phil.  Mag.  27,  p  632,  1914)  found  that  in  certain 
reactions  the  solutions  passed  through  intermediate  phases  which 
could  be  traced  by  means  of  changes  in  the  absorption  bands.  A 
great  many  electrolytes,  on  going  into  solution,  react  chemically  with 
water.  For  instance,  when  ferric  chloride  goes  into  aqueous  solution, 
one  or  more  of  the  chlorine  atoms  may  be  replaced  by  OH  radicals 
from  the  water  with  the  formation  of  a  basic  ferric  salt  or  the  ferric 
hydroxide  and  hydrochloric  acid.  The  ferric  hydroxide  is  brick  red 
in  color.  A  colorless  salt,  as  aluminum  chloride,  when  added  to  the 
above  solution,  may  have  a  catalytic  effect  in  bringing  about  these 
reactions.  Furthermore,  the  effect  would  undoubtedly  be  increased 
by  heating,  as  hydrolysis  is  promoted  by  rise  of  temperature. 

Jones's  work  in  the  visible  and  ultra-violet  was  done  by  the 
method  of  spectro-photography.  This  method,  while  possessing  many 
advantages,  has  the  disadvantage  that  the  results  are  qualitative  only. 
Changes  in  the  width  and  position  of  an  absorption  band  can  be 
measured,  and  some  notion  of  the  relative  intensities  of  different 
bands  can  be  obtained  from  the  blackness  of  the  photographs,  but  no 
information  concerning  the  shape  of  the  absorption  curve  is  possible. 

The  plan  of  the  present  work  was  to  repeat  some  of  the  work 
of  Jones,  making  use  of  the  method  of  spectro-photometry,  thus 
making  the  results  quantitative,  and  to  extend  the  work  to  other 
salts.  In  particular  it  was  desired  to  study  the  effect  of  those  color- 
less salts  which  do  not  form  hydrates,  upon  the  absorption  of  colored 
salts,  and  the  effect  of  one  colored  salt  upon  another.  It  was  also 
planned  to  study  the  effect  of  temperature  on  solutions  of  certain 
salts  and  mixtures  of  different  salts.  In  addition,  it  was  desired  to 
measure  the  absorption  of  certain  organic  compounds.  No  attempt 
has  been  made  to  investigate  Beer's  law. 


12 

APPARATUS  AND  METHOD. 

The  method  of  the  present  work  was  that  of  spectro-photometry. 
The  results  are  quantitative,  although  the  method  does  not  adapt 
itself  to  a  high  degre  of  precision.  The  instrument  consists  of  a 
Hilger  spectrometer  with  a  Nutting  photometer  box.  A  sketch  of  the 
apparatus,  showing  in  detail  the  optical  parts  of  the  photometer  box, 
is  given  in  plate  1 .  The  light  source  is  at  S.  A  40  watt  tungsten  lamp 
was  used.  A  Nernst  glower  or  other  line  source  might  have  been 
better,  but  the  lamp  used  gave  very  good  results.  In  the  earlier  part 
of  the  work  direct  current  from  the  storage  battery  was  used  for 
illumination,  but  later  on  ordinary  A.  C.  power  was  used.  The  ap- 
paratus functioned  about  as  well  in  the  latter  case  as  in  the  former. 
Two  beams  of  light,  rendered  parallel  by  the  wedge-shaped  prisms, 
pass  through  the  photometer  box.  E  is  a  glass  cell  about  1  cm.  thick, 
2  cms.  wide  by  2J/2  cms.  deep  which  contained  the  solution  whose 
absorption  was  to  be  measured.  D  is  an  exactly  similar  cell  which  in 
general  contained  either  distilled  water  or  an  aqueous  solution  of  a 
colorless  salt. 

In  order  to  obtain  even  fair  results,  certain  precautions  were 
found  to  be  necessary.  The  illumination  must  be  neither  too  faint  nor 
too  brilliant.  At  either  extremity  of  the  spectrum  the  illumination 
was  too  feeble  for  good  results.  Where  the  illumination  was  too  in- 
tense, it  was  cut  down  by  means  of  a  variable  resistance.  The  best 
results  were  obtained  with  a  rather  faintly  illuminated  field.  The  eye 
should  be  in  good  condition,  rested,  and  not  recently  exposed  to  a 
strong  light.  It  is  useless  to  try  to  take  readings  immediately  after  a 
walk  in  the  bright  sunlight.  As  far  as  possible  all  light  was  eliminated 
except  that  passing  through  the  telescope.  The  heating  effect  from 
the  lamp  was  largely  eliminated  by  placing  a  large  cell  full  of  water 
in  front  of  the  cells  D  and  E.  This  cell  was  broad  and  deep  and 
about  3cms.  thick.  Considerable  care  was  taken  to  keep  the  solution 
free  from  undissolved  particles  in  suspension.  If,  on  close  inspection, 
there  was  the  slightest  indication  of  murkiness,  the  solution  was 
filtered.  In  the  case  of  the  salts  of  chromium,  the  filtration  was  re- 
peated many  times.  The  wave-length  readings  on  the  drum  was 
occasionally  checked  against  a  known  wave-length,  as  the  prism  P 
sometimes  became  slightly  displaced. 

To  obtain  the  absorption  of  a  simple   solution   the   following 


13 

method  was  used:  a  definite  weighed  quantity  of  the  salt  was  dis- 
solved in  distilled  water  and  then  water  added  to  make  a  definite 
volume  of  the  solution.  This  same  concentration  was  always  used 
when  dealing  with  the  same  colored  salt.  The  cell  E  was  filled  with 
the  colored  solution.  The  cell  D  was  filled  with  distilled  water.  The 
interval  between  the  readings  ordinarily  vairied  from  50  to  200  A.  U. 
The  number  of  readings  for  any  given  wave  length  varied  from  three 
to  eight. 

To  measure  the  absorption  of  a  mixed  solution  of  a  colored  salt 
with  a  colorless  one,  the  following  plan  was  used:  the  two  salts, 
after  having  been  weighed,  were  mixed  together.  Sufficient  distilled 
water  was  added  to  dissolve  both  salts.  Then  water  was  added  in 
sufficient  quantity  to  make  the  desired  volume  of  solution.  The  cell 
E  was  filled  with  this  solution.  An  aqueous  solution  of  the  colorless 
salt  was  then  made  by  dissolving  the  same  quantity  of  the  colorless 
salt  in  the  same  volume  of  solution.  This  solution  was  placed  in  the 
Cell  D. 

In  the  case  of  two  clored  salts,  as  for  instance  cobalt  chloride 
and  copper  chloride,  the  procedure  was  as  follows :  the  solution  of  the 
two  salts,  each  having  the  same  concentration  as  when  its  absorption 
was  determined  alone  (that  is,  the  same  number  of  molecules  per 
unit  volume),  was  placed  in  the  cell  E.  In  the  cell  D  was  placed  an 
aquous  solution  of  copper  chloride  of  the  same  concentration  as  that 
in  the  ceil  E.  The  absorption  of  the  copper  chloride  in  the  cell  D 
should  balance  that  of  the  same  salt  in  the  cell  E.  The  readings  should 
then  indicate  only  the  absorption  of  the  cobalt  chloride  plus  whatever 
change  was  brought  about  by  the  influence  of  the  one  salt  upon 
the  other. 

When  dealing  with  a  mixture  of  three  colored  salts,  still  another 
plan  was  followed.  The  solution  of  the  three  salts,  each  having  the 
same  concentration  as  before,  was  placed  in  the  cell  E.  In  D  was 
placed  distilled  water.  The  absorption  was  measured  throughout  the 
visible  spectrum  and  the  curve  plotted.  But  in  this  case  the  extinction 
coefficient  was  not  determined,  the  extinction  coefficient  of  a  mixture 
apparently  having  no  meaning.  The  curve  was  plotted  in  instrument 
readings;  that  is,  in  terms  of  the  logarithm  of  the  reciprocal  of  the 
square  of  the  cosin  of  the  angle  of  rotation  of  the  analyser.  The 
comparison  curve  was  obtained  by  adding  together  the  absorption 
curves  of  the  three  salts  as  determined  separately.  The  difference 


14 

between  these  two  curves  shows  the  changes  due  to  the  influence  of  the 
different  salts  on  each  other.  Two  sets  of  readings  are  given  in 
Table  I,  copied  directly  from  the  original  records.  In  the  first 
column  are  recorded  wave-lengths  in  millimicrons  and  in  the  second 
the  corresponding  readings  of  the  instrument.  It  may  be  observed 
that  where  the  solution  is  nearly  transparent,  and  therefore  the  read- 
ings very  small,  the  percentage  of  error  is  liable  to  be  quite  large. 

CONCENTRATIONS. 

In  this  series  of  experiments  the  same  concentration  was  always 
used  with  the  same  colored  salt.  In  preparing  the  solution,  the  salt 
was  always  weighed  to  within  one  tenth  of  one  per  cent.  Consider- 
able care  was  taken  to  free  the  salt  from  excess  moisture.  The  con- 
centration was  determined  in  gram  molecules  per  liter,  and  are  as 
follows:  Copper  chloride,  .2742;  copper  nitrate,  .1645;  copper 
sulphate,  .1720;  nickel  chloride,  .342;  nickel  nitrate,  .2286;  nickel 
sulphate,  .3202 ;  cobalt  chloride,  .  1 1 39 ;  cobalt  nitrate,  .  1 600 ;  cobalt 
sulphate,  .2025;  ferric  chloride,  .1560;  ferric  nitrate,  .085; 
chromium  chloride,  .03396;  chromium  nitrate,  .01067;  uranyl 
chloride,  .727.  The  concentration  of  the  colorless  salts  was  not  kept 
strictly  constant  throughout,  and  will  be  taken  up  in  the  discussion 
of  individual  cases. 

MIXTURE  OF  COPPER  CHLORIDE  WITH  OTHER  CHLORIDES. 

With  cerium  chloride.  The  concentration  of  the  cerium  chloride 
was  .60.  The  curves  are  shown  on  plate  2-e.  In  all  these  curves  the 
abscissa  represents  wave-length  in  milli-microns  and  in  nearly  all  of 
them  the  ordinate  represents  absorption  in  terms  of  the  extinction 
coefficient.  The  various  sets  of  curves  are  displaced  the  one  above 
the  others  at  convenient  distances  so  that  while  the  origin  of 
abscissas  is  the  same,  the  origin  of  the  ordinates  is  different  for  each 
set.  Unless  otherwise  specified  the  dotted  line  represents  the  ab- 
sorption of  the  colored  salt  alone,  while  the  full-line  curve  represents 
that  of  the  mixture.  In  this  case  no  very  great  effct  is  shown,  although 
there  is  increased  absorption  in  the  red. 

With  zinc  chloride,  plate  2-d.  Concentration  of  the  zinc  chlor- 
ide=  1 .50.  Increased  absorption  in  the  ired. 

With  sodium  chloride,  plate  2-c.     Concentration  of  the  sodium 


15 

chloride=2.8.  Increased  absorption  in  the  violet,  decrease  from 
wave-length  530  to  615,  and  marked  increase  in  the  red.  The  equa- 
tion of  the  curve  has  been  determined  as  shown  in  the  graph. 

With  potassium  chloride,  plate  2-b.  Concentration  of  potas- 
sium chloride =2. 2.  Marked  increase  in  absorption  in  the  red.  The 
curve  is  found  to  follow  an  exponential  law,  as  shown  in  the  graph. 

With  ammonium  chloride,  plate  2-a.  Concentration  of  am- 
mounium  chloride  =  3.1.  The  mixture  shows  very  great  absorption 
in  the  violet,  although  the  copper  chloride  is  transparent  in  that  region 
of  the  spectrum.  There  is  decrease  in  the  green  and  yellow  and  in- 
crease in  the  red.  The  equation  has  been  determined  and  found  to 
be  expotential. 

With  ferric  chloride,  plate  6-a.     No  very  pronounced  influence. 

COBALT  CHLORIDE  WITH  OTHER  CHLORIDES. 

With  sodium  chloride,  plate  3-e.  Concentration  of  sodium 
chloride  =  2.8.  Very  considerable  increase  in  the  green. 

With  potassium  chloride,  plate  3-d.  Concentration  of  KC1  =  2.2. 
Very  greatly  increased  absorption  in  the  green. 

With  cerium  chloride,  plate  3-c.  Concentration  of  the  cerium 
chloride  =  .60.  Increased  absorption  throughout  the  spectrum. 

Cobalt  chloride  with  zinc  chloride,  plate  3-b.  Concentration  of 
zinc  chloride  =  1.5.  Considerable  increase  in  the  green;  increase 
throughout  except  in  the  yellow. 

With  ammonium  chloride,  plate  3-a.  Concentration  of  am- 
monium chloride  =  2.8.  Very  great  increase  in  absorption  in  the 
green.  It  may  be  noticed  that  ammonium  chloride,  which  does  not 
form  hydrates,  produces  a  greater  change  in  the  absorption  of  cobalt 
chloride  than  either  zinc  chloride,  sodium  chloride,  or  lithium  chloride, 
all  of  which  form  hydrates. 

With  lithium  chloride,  plate  16-b.  Concentration  of  lithium 
chloride  =  3.9.  Considerable  absorption  except  in  the  violet  and  blue. 

With  ferric  chloride,  plate  4-a.  Marked  increase  in  absorption 
in  the  blue  and  considerable  increase  in  the  region  of  the  long  wave 
lengths. 

With  chromium  chloride,  plate  4-b.  Broadening  of  the  principle 
absorption  band  in  the  blue  and  green,  and  increased  absorption  in 
the  red. 


16 

With  nickel  chloride,  plate  4-c.  Increased  absorption  through- 
out, but  especially  at  the  red  end  of  the  spectrum. 

With  copper  chloride,  plate  4-d.  Decrease  in  the  yellow  and 
increase  in  the  red. 

NICKEL  CHLODIDE  WITH  OTHER  CHLORIDES. 

With  sodium  chloride,  plate  5-d.  Concentration  of  sodium 
chloride  =  2.75.  Decrease  in  absorption  throughout  the  visible 
spectrum. 

With  potassium  chloride,  plate  5-c.  Concentration  of  potassium 
chloride  =  2.00.  Marked  decrease  in  absorption  for  the  middle  of 
the  spectrum. 

With  cerium  chloride,  plate  5-b.  Concentration  of  cerium 
chloride  =  .60.  Marked  increase  in  transparency  in  the  blue,  green 
and  orange. 

With  lithium  chloride,  plate  5-e.  Concentration  of  LiCl  —  4.0. 
There  is  a  very  great  increase  in  absorption  throughout  the  spectrum. 

With  copper  chloride,  plate  6-c. .  There  is  a  rather  large  in- 
crease in  absorption  in  the  gnreen. 

With  ferric  chloride,  plate  6-b.    Enormous  increase  in  the  violet. 

AQUEOUS  SOLUTIONS  OF  FERRIC  CHLORIDE 
WITH  OTHER  CHLORIDES. 

With  lithium  chloride,  plate  7-c.  Concentration  of  LiCl  = :  1 .8. 
Greatly  increased  absorption  in  the  region  of  the  short  wave  lengths. 

With  calcium  chloride,  plate  7-b.  Concentration  of  calcium 
chloride  =  1 .00.  Great  increase  in  absorption  in  the  violet  and  blue. 

With  aluminum  chloride,  plate  7-a.  Concentration  of  aluminum 
chloride  =  0.340.  This  is  the  most  striking  example  of  color  change 
produced  by  the  addition  of  a  colorless  salt  to  a  colored  one  that  has 
been  met  with  in  this  series  of  experiments.  The  change  in  color  is 
from  that  of  a  clear  amber  to  a  brick  red.  Undoubtedly  there  is 
formation  of  ferric  hydroxide. 

With  ammonium  chloride,  plate  8-a.  Concentration  of  the  am- 
monium chloride  =  2.80.  There  is  tremendous  increase  in  absorption 
in  the  violet  and  blue,  with  considerable  decrease  in  the  green.  At 
wave-length  480  the  increases  is  more  than  200%.  This  curve  is 
found  to  be  exponential,  as  shown  in  the  graph. 


17 

With  zinc  chloride,  plate  8-b.  Concentration  of  zinc  chloride 
=  1 .30.  Enormous  increase  in  absorption  in  the  violet,  and  decrease 
in  the  green.  The  curve  is  exponential  in  form. 

With  sodium  chloride,  plate  9-c.  Concentration  of  NaCl  =  2.60. 
There  is  enormous  increase  in  absorption  for  the  shorter  wave  lengths. 

With  potassium  chloride,  plate  9-b.  Concentration  of  KC1=:2.00. 
Great  increase  in  absorption  in  the  violet  and  blue. 

With  cerium  chloride,  plate  9-a.  Concentration  of  cerium  chloride 
=  0.60.  There  is  very  great  increase  in  absorption  in  the  violet. 

MIXTURES  OF  CHROMIUM  CHLORIDE  WITH 
OTHER  CHLORIDES. 

With  sodium  chloride,  plate  10-f.  Concentration  of  NaCl=3.00. 
There  is  increased  absorption  throughout  the  spectrum. 

With  potassium  chloride,  plate  10-e.  Concentration  of  KC1  == 
2.50.  There  is  increased  absorption  throughout  the  spectrum. 

With  aluminium  chloride,  plate  10-d.  Concentration  of  the 
aluminium  chloride  =  0.30.  There  is  increase  in  the  absorption  up  to 
wave-length  590,  beyond  which  there  is  decrease. 

With  calcium  chloride,  plate  1 0-c.  Concentration  of  the  calcium 
chloride  =  1 .50.  There  is  scarcely  any  change  in  absorption.  This 
fact  seems  to  be  widely  at  variance  with  the  solvate  theory,  for 
calcium  chloride,  which  has  an  enormous  affinity  for  water,  should 
produce  a  great  change  in  the  absorption. 

With  zinc  chloride,  plate  10-b.  Concentration  of  zinc  chloride 
=  1 .60.  No  very  decided  change  in  absorption. 

With  ammonium  chloride,  plate  10-a.  Concentration  of  am- 
monium chloride  —  2.8.  Decreased  absorption  throughout. 

With  copper  chloride,  plate  1 1  -d.  There  is  increase  in  absorp- 
tion for  the  shorter  wave-lengths. 

With  nickel  chloride,  plate  1 1  -c.  There  is  increase  in  absorp- 
tion in  the  violet  and  decrease  from  wave-length  540  to  660. 

With  ferric  chloride,  plate  1 1  -b.  There  is  increase  in  absorption 
throughout. 

With  cerium  chloride,  plate  1 1-a.  There  is  scarcely  any  change 
in  absorption. 


18 

URYANYL  CHLORIDE  WITH  OTHER  CHLORIDES. 

With  cerium  chloride,  plate  1 2-e.  The  curves  show  considerable 
increase  in  the  violet. 

With  potassium  chloride,  plate  1 2-d.  Concentration  of  KC1=2.30. 
There  is  very  greatly  increased  absorption  in  the  violet  and  blue. 

With  aluminium  chloride,  plate  12-c.  Concentration  of  alum- 
inum chloride  =  0.30.  There  is  great  increase  in  absorption  for  the 
shorter  wave  lengths. 

With  zinc  chloride,  plate  12-b.  Concentration  of  zinc  chloride 
—  1.60.  There  is  decrease  in  absorption  throughout. 

With  sodium  chloride,  plate  1 2-a.  Concentration  of  NaCl=3.00. 
The  curve  shows  very  great  increase  in  absorption  for  the  shorter 
wave  lengths. 

With  copper  chloride,  plate  13-d.  There  is  very  little  change 
except  in  the  red  where  there  is  considerable  increase  in  absorption. 

With  nickel  chloride,  plate  13-c.  There  is  no  very  decided 
change  in  absorption. 

With  cobalt  chloride,  plate  13-b.  There  seems  to  be  a  slight 
increase  in  absorption  for  most  of  the  visible  spectrum. 

With  calcium  chloride,  plate  13-a.  Concentration  of  calcium 
chloride  =  1.50.  There  is  a  very  lairge  and  general  increase  in 
absorption  throughout  the  spectrum. 

MIXTURES  OF  THREE  COLORED  CHLORIDES. 

Copper  chloride,  cobalt  chloride  and  chromium  chloride,  plate 
14-c.  There  is  increase  in  absorption  from  440  to  550,  decrease  from 
550  to  670  and  increase  the  Test  of  the  way. 

Copper  chloride,  nickel  chloride,  and  chromium  chloride,  plate 
1 4-b.  Just  as  in  the  preceding  case  there  is  increased  absorption  from 
440  to  550,  decreased  absorption  from  550  to  650  and  an  increase 
from  650  to  the  end  of  the  visible  spectrum. 

Copper  chloride,  ferric  chloride,  and  sodium  chloride,  plate  1 4-a. 
There  is  an  enormous  increase  in  absorption  at  both  ends  of  the 
spectrum  with  no  change  whatever  in  the  center.  An  exponential 
curve  was  determined  which  fits  the  experimental  curve  almost 
prfectly. 

Copper  chloride,  cobalt  chloride,  and  ferric  chloride,  plate  1 5-a. 


19 

There  is  considerable  increase  in  absorption  at  both  ends  of  the  spec- 
trum, with  a  rather  marked  decrease  from  560  to  640. 

Copper  chloride,  chromium  chloride,  and  ferric  chloride,  plate 
1 5-b.  There  is  very  great  increase  in  the  violet,  considerable  increase 
in  the  blue,  decrease  in  the  yellow,  and  increase  in  the  red. 

Cobalt  chloride,  nickel  chloride  and  chromium  chloride,  plate 
15-c.  There  is  considerable  decrease  in  absorption  between  wave- 
lengths 540  and  640. 

Chromium  chloride,  cobalt  chloride,  and  ferric  chloride,  place 
16-a.  There  is  rather  marked  increase  in  absorption  for  the  shorter 
wave  lengths. 

MIXTURES  OF  COPPER  NITRATE  WITH  OTHER  NITRATES. 

With  aluminum  nitrate,  plate  1 7-c.  Concentration  of  aluminum 
nitrate  was  .44.  The  mixed  solution  is  more  transparent  than  that 
of  copper  nitrate  for  practically  all  of  the  visible  spectrum. 

With  calcium  nitrate,  plate  17-b.  The  concentration  of  calcium 
nitrace  was  0.70.  The  mixed  solution  is  more  transparent  than  the 
solution  of  copper  nitrate  from  wave-length  540  to  640,  while  for 
longer  wave-lengths  it  is  more  opaque. 

With  sodium  nitrate,  plate  17-a.  The  influence  of  the  sodium 
nitrate  manifests  itself  as  an  increase  in  transparency  between  wave- 
length 540  and  640. 

With  lithium  nitrate,  plate  17-d.  Concentration  of  lithium 
nitrate  =  2.40.  The  change  consists  in  an  increase  in  absorption 
from  wave-length  560  to  the  red  end  of  the  visible  spectrum. 

With  ammonium  nitrate,  plate  1 7-e.  Concentration  of  the  am- 
monium nitrate  =  1.85.  There  is  increased  absorption  in  the  region 
of  the  longer  wave-lengths. 

COBALT  NITRATE  WITH  OTHER  NITRATES. 

With  ammonium  nitrate,  plate  18-a.  Concentration  of  ammon- 
ium nitrate  =  1.85.  The  influence  of  the  ammonium  nitrate  mani- 
fests itself  as  an  increase  in  transparency  throughout  the  visible  spec- 
trum, the  greatest  change  being  at  about  wave-length  600. 

With  sodium  nitrate,  plate  18-b.  Concentration  of  sodium 
nitrate  =  1.90.  There  is  decrease  in  absorption  from  wave-length 
480  to  the  red  end  of  the  spectrum. 


20 

With  calcium  nitrate,  plate  18-c.  Concentration  of  calcium 
nitrate  =  1.50.  It  will  be  seen  that  there  is  a  general  increase  in 
transparency  throughout  the  spectrum. 

With  aluminum  nitrate,  plate  18-d.  Concentration  of  aluminum 
nitrate  =  .440.  There  is  a  slight  decrease  in  absorption  throughout 
the  spectrum. 

With  lithium  nitrate,  plate  19-a.  Concentration  of  lithium 
nitrate  ==  2.400.  There  is  some  increase  in  absorption  in  the  violet, 
blue,  green,  and  red. 

With  copper  nitrate,  plate  21 -a.  There  is  greatly  increased 
transparency  from  wave-length  570  to  the  red  end  of  the  spectrum. 

MIXTURES  OF  NICKEL  NITRATE  WITH  OTHER  NITRATES. 

With  ammonium  nitrate,  plate  20-a.  Concentration  of  am- 
monium nitrate  —  1.850.  It  will  be  observed  that  there  is  a  re- 
markable increase  in  transparency  throughout  the  spectrum.  The 
mixed  solution  is  transparent  as  water  to  wave-length  580. 

With  calcium  nitrate,  plate  20-b.  Concentration  of  calcium 
nitrate  =  .700.  There  is  a  general  decrease  in  absorption  except  for 
a  small  region  in  the  green. 

With  aluminum  nitrate,  plate  20-c.  Concentration  of  aluminum 
nitrate  =  .440.  There  is  a  considerable  increase  in  absorption  in  the 
green  and  decrease  in  the  red. 

With  lithium  nitrate,  plate  20-d.  Concentration  of  lithium 
nitrate  =  2.40.  The  only  striking  change  is  an  increase  in  trans- 
parency from  wave-length  550  to  640. 

With  sodium  nitrate,  plate  20-e.  Concentration  of  sodium  nitrate 
=  1.90.  Increased  transparency  in  the  yellow  and  red. 

With  cobalt  nitrate,  plate  21-b.  There  is  an  enormous  increase 
in  transparency  for  almost  the  entire  length  of  the  spectrum. 

With  copper  nitrate,  plate  21-c.  The  curves  show  decrease  in 
absorption  in  the  blue,  increase  in  the  green,  decrease  in  the  yellow 
and  enormous  decrease  in  the  red. 

MIXTURES  OF  FERRIC  NITRATE  WITH  OTHER  NITRATES. 

With  calcium  nitrate,  plate  22-a.  Concentration  of  calcium 
nitrate  =  .700.  There  is  marked  increase  in  absorption  in  the  region 
of  the  shorter  wave-lengths. 


21 

With  lithium  nitrate,  plate  22-b.  Concentration  of  lithium 
nitrate  =  2.40.  There  is  very  little  change  in  absorption  except  from 
wave-length  520  to  610,  where  there  is  increased  transparency, 

With  sodium  nitrate,  plate  22-c.  Concentration  of  sodium  nitrate 
—  1 .90.  There  is  considerable  increase  in  transparency  in  the  middle 
of  the  spectrum  and  a  general  but  smaller  increase  for  the  longer 
wave-lengths. 

With  aluminium  nitrate,  plate  22-d.  Concentration  of  aluminum 
nitrate  =  .440.  Slightly  increased  transparency  throughout. 

With  ammonium  nitrate,  plate  22-e.  Concentration  of  am- 
monium nitrate  =  1.850.  Very  great  increase  in  transparency 
throughout  the  visible  spectrum. 

With  cobalt  nitrate,  plate  23-a.  There  is  enormous  increase  in 
transparency  especially  in  the  middle  of  the  spectrum. 

With  nickel  nitrate.,  plate  23-b.  There  is  a  very  great  increase 
in  transparency  at  about  wave-length  600,  and  a  smaller  general 
increase  throughout. 

With  copper  nitrate,  plate  23-c.  There  is  very  great  increase 
in  transparency  from  wave-length  540  to  660,  and  a  smaller  increase 
in  the  region  of  the  shorter  wave-lengths. 

MIXTURES  OF  CHROMIUM  NITRATE  WITH  OTHER  NITRATES. 

With  aluminium  nitrate,  plate  24-a.  Concentration  of  aluminum 
nitrate  =  0.440.  There  is  very  great  decrease  in  absorption  in  the 
violet,  the  decrease  growing  gradually  smaller  as  the  wave-length 
increases. 

With  calcium  nitrate,  plate  24-b.  Concentration  of  calcium 
nitrate  =  0.500.  There  is  considerable  increase  in  transparency  in 
the  region  of  the  shorter  wave-lengths. 

With  lithium  nitrate,  plate  24-c.  Concentration  of  lithium  ni- 
trate —  2.400.  There  is  great  decrease  in  absorption  in  the  violet, 
blue,  and  green. 

With  sodium  nitrate,  plate  24-d.  Concentration  of  sodium  ni- 
trate =  2.300.  The  surves  show  decrease  in  absorption  in  the  region 
of  the  shorter  wave-lengths. 

With  cobalt  nitrate,  plate  25-a.  There  is  a  general  and  very 
great  decrease  in  absorption,  the  greatest  effect  being  found  in  the 
region  of  the  shorter  wave-lengths. 


22 

With  nickel  nitrate,  plate  25-b.  The  decrease  in  absorption  is 
very  pronounced  throughout  the  visible  spectrum. 

With  copper  nitrate,  plate  25-c.  Very  great  increase  in  trans- 
parency is  shown  throughout  the  spectrum. 

DIDYMIUM  NITRATE  WITH  ALUMINIUM  NITRATE. 

Two  and  728-1000  grams  of  didymium  nitrate  were  dissolved  in 
sufficient  distilled  water  to  make  7.5  c.  c.  of  solution.  The  concentra- 
tion of  the  aluminium  nitrate  was  0.400.  The  absorption  was  meas- 
ured and  the  curve  plotted  as  shown  in  plate  36-b.  The  aluminium 
nitrate  seems  to  produce  no  appreciable  effect  on  the  absorption  of 
the  didymium  nitrate.  The  curve  of  the  mixture  practically  coincides 
with  that  of  the  simple  salt.  It  will  be  noticed  that  there  are  many 
sharp  bands,  the  most  prominent  having  their  maxima  at  about  wave- 
lengths 445,423,  578.5,  and  744.  The  extinction  coefficient  was 
not  plotted.  The  ordinates  are  plotted  in  terms  of  the  readings  of  the 
instrument. 

MIXTURES  OF  THREE  COLORED  NITRATES. 

Copper  nitrate,  nickel  nitrate,  and  ferric  nitrate,  plate  26-a. 
There  is  a  small  decrease  in  absorption  in  the  violet,  blue,  and  green, 
and  a  very  pronounced  decrease  for  the  remainder  of  the  spectrum. 

Copper  nitrate,  cobalt  nitrate,  and  ferric  nitrate,  plate  26-b. 
There  is  greatly  increased  transparency  throughout,  the  greatest  change 
coming  in  the  middle  of  the  spectrum. 

Nickel  nitrate,  cobalt  nitrate,  and  ferric  nitrate,  plate  26-c. 
Again  there  is  a  very  great  decrease  in  absorption  throughout  the  spec- 
trum, with  the  greatest  decrease  coming  at  about  wave-length  580. 

MIXTURES  OF  COPPER  SULPHATE  WITH  OTHER  SULPHATES. 

With  aluminium  sulphate,  plate  27-a.  The  absorption  curve 
shows  very  little  deviation  from  that  of  the  copper  sulphate  alone. 
Concentration  of  aluminium  sulphate  =  0.270. 

With  nickel  sulphate,  plate  27-b.  There  is  considerable  decrease 
in  absorption  for  the  longer  wave-lengths. 

With  cobalt  sulphate,  plate  27-c.  There  is  increase  in  absorp- 
tion in  the  violet  and  blue,  and  decrease  beyond  wave-length  560. 

With  postasshim  sulphate,  plate  27-d.     Concentration  of  potas- 


23 

sium  sulphate  =  0.5 1 0.     There  is  no  noticeable  change  in  absorption 
whatever. 

With  ammonium  sulphate,  plate  27-e.  Concentration  of  am- 
tionmonium  sulphate  =  1 .500.  There  is  no  very  pronounced  change 
in  absorption. 

MIXTURES  OF  COBALT  SULPHATE  WITH  OTHER  SULPHATES. 

With  aluminium  sulphate,  plate  28-a. .  .Concentration  of  alu- 
minium sulphate  =  0.270.  The  curves  show  some  increase  in  absorp- 
tion in  the  violet  and  blue  and  some  decrease  in  the  red. 

With  ammonium  sulphate,  plate  28-b.  Concentration  of  am- 
monium sulphate  =  1.250.  There  is  slight  increase  in  the  violet 
and  blue. 

With  nickel  sulphate,  plate  28-c.  There  is  a  slight  increase  for 
the  shorter  wave-lengths  and  decrease  for  the  longer. 

With  potassium  sulphate,  plate  28-d.  Concentration  of  potas- 
sium sulphate  =  0.510.  There  is  a  general  increase  in  absorption 
throughout  the  spectrum. 

THE  EFFECT  OF  TEMPERATURE  UPON  ABSORPTION. 

Far  the  investigation  of  the  effect  of  temperature  upon  absorp- 
tion, an  electric  furnace  of  inside  dimensions  1 .9  x  8.6  x  5.7  cms.  was 
constructed.  It  was  made  of  brass  and  covered  with  two  or  three 
layers  of  asbestos  paper  over  which  was  wound  about  20  feet  of  man- 
ganin  wire  having  a  total  resistance  of  about  60  ohms.  This  in  turn 
was  covered  with  a  paste  made  of  magnesium  oxide  and  sodium  sili- 
cate, so  that  the  total  thickness  of  covering  was  about  one  centimeter. 
In  order  to  allow  the  light  to  pass  through,  two  holes  of  1 .8  cm.  diam- 
eter were  bored  through  each  side.  A  wooden  lid,  boiled  in  beeswax 
and  containing  a  hole  for  the  insertion  of  a  mercury  thermometer  was 
made  for  the  furnace. 

About  this  time  the  glass  cells  used  as  containers  for  the  solutions 
went  to  pieces.  Some  of  the  solutions  attacked  the  cement  of  the 
cells,  thus  setting  up  strains  which  caused  the  glass  to  break.  A  gnreat 
number  of  cements  were  tried  and  found  useless.  A  brass  cell  was 
then  constructed  and  gold-plated  on  the  inside  with  glass  plates 
fastened  on  the  sides  by  mechanical  means.  The  hot  solutions  at- 
tacked the  brass  through  the  gold,  probably  for  the  reason  that  the 


24 

gold  plating  was  not  thick  enough  or  not  uniformly  distributed. 
Finally  the  inside  of  the  cell  was  coated  with  de  Khotinsky  wax.  This 
resisted  the  action  of  the  aqueous  solution  perfectly  and  also  proved  to 
be  a  good  cement  for  fastening  the  parts  of  the  cell  together.  Some 
of  the  readings  are  shown  in  tables  III  and  IV.  These  values  are 
copied  from  the  book  just  as  they  were  recorded  at  the  time  of  tak- 
ing the  observations.  In  the  curves  the  absorption  at  the  higher  tem- 
perature is  always  represented  by  the  full  line  curve,  while  that  at 
room  temperature  is  shown  by  the  dotted  curve. 

MEASUREMENT  OF  TEMPERATURE  EFFECT  UPON 
VARIOUS  SOLUTIONS 

Copper  chloride,  plate  34-d.  Readings  were  taken  at  23.5° 
C,  46.7°,  and  66.0°  C.  The  increase  in  absorption  extends  from  wave- 
length 500  to  the  red  end  of  the  spectrum. 

Copper  chloride  with  ammonium  chloride,  plate  29-a.  The  higher 
temperature  was  67.0°  C.  There  is  tremendous  increase  in  absorption 
in  the  violet  and  blue,  although  neither  the  copper  chloride  nor  the 
ammonium  chloride  shows  any  temperature  effect  in  that  region. 
There  is  also  very  great  increase  in  the  red. 

The  equations  of  these  curves  have  been  determined  as  shown  in 
the  graph.  For  the  full  line  curve,  between  wave-lengths  460  and 
530  the  b  of  the  formula  has  the  value  -.05128  and  the  k  of  the 
formula  5.956.  Between  530  and  610,  b=.0296  and  k=.1479. 
Between  610  and  the  ored  end  of  the  spectrum,  b  =  .02257  and 
k  =  1.706.  For  the  dotted  line,  from  wave-length  440  to  500, 
b  =  -.04582  and  k  =  5.83.  From  560  to  the  red  end  of  the  spec- 
trum, b  =  .01946  and  k  =  .4678. 

Copper  chloride  with  sodium  chloride,  plate  29-b.  The  higher 
temperature  was  at  58.5°  C.  The  mixed  solution  shows  tremendous 
increase  in  absorption  in  the  violet,  although  neither  of  the  simple 
solutions  of  which  the  mixture  is  composed  shows  any  temperature 
effect  in  that  iregion.  There  is  increased  absorption  throughout  the 
spectrum.  The  equations  of  these  curves  have  been  determined  as 
shown  in  the  graph.  For  the  full  line  curve  between  the  wave-lengths 
440  and  520  the  b  of  the  formula  =  -.04552  and  k  =  7. 1 3.  From 
wave-length  520  to  the  red  end  of  the  spectrum,  b  =  .02256  and 
k=.2144. 

Copper  chloride  with  potassium    chlroide,    plate    29-c.     The 


25 

higher  temperature  was  69.6°  C.  There  is  a  remarkable  increase  in 
the  violet  and  a  smaller  increase  throughout  the  spectrum.  The 
equation  of  the  curves  have  been  determined  as  shown  in  the  graphs. 

Cobalt  chloride,  plate  30-a.  The  higher  temperature  was  68.5  °C. 
There  is  great  increase  in  absorption  throughout  the  spectrum. 

Ferric  chloride,  plate  32-c.  Temperatures  were  23.5°  and 
5 1 .0°  C.  There  is  very  great  increase  in  absorption  for  the  shorter 
wave-lengths. 

Ferric  chloride  with  ammonium  chloride,  plate  33-a.  Tempera- 
tures were  23.5°  and  49.0°  C.  There  is  very  great  increase  in  ab- 
sorption for  nearly  the  entire  length  of  the  spectrum. 

Ferric  chloride  with  potassium  chloride,  plate  33-b.  Tempera- 
tures were  23.5°  and  52.0°  C.  The  curves  show  tremendous  increase 
in  absorption  for  the  entire  spectrum. 

Ferric  chloride  with  sodium  chloride,  plate  33-c.  Temperatures 
were  23.5°  and  48.0°  C.  Tremendous  increase  in  absorption  for  the 
shorter  wave-lengths. 

Nickel  chloride.  The  temperatures  were  23.5°  and  69.7°  C. 
There  was  no  change  in  the  absorption  whatever  and  the  curve  was 
not  plotted. 

Nickel  chloride  wtih  sodium  chloride,  plate  34-a.  The  tempera- 
tures were  23.5°  and  68.0°  C.  In  the  violet  and  blue  the  increase  in 
absorption  is  400  to  600%.  There  is  great  increase  throughout  the 
spectrum. 

Chromium  chloride.  The  temperatures  were  23.5°  and  67.1  °  C. 
No  change  in  absorption  could  be  detected  and  the  curve  was  not 
plotted. 

Chromium  chloride  with  potassium  chloride,  plate  35-c.  Tem- 
peratures were  23.5°  and  73.0°  C.  There  is  a  rather  large  and  gen- 
eral increase  in  absorption  throughout  the  spectrum,  although  the 
chromium  chloride  alone  shows  no  increase  and  potassium  chloride  is 
transparent. 

Neodymium  chloride,  plate  36-a.  Temperatures  were  23.5° 
and  73.0°  C.  It  will  be  seen  that  there  is  very  great  increase  in  ab- 
sorption in  the  violet  and  blue,  and  again  in  the  yellow  and  red.  For 
the  middle  of  the  spectrum  there  is  no  apparent  change. 

Ferric  nitrate,  plate  31 -a.  Temperatures  were  23.5°  and  58.2° 
C.  There  is  very  great  increase  in  absorption  in  the  region  of  the 
shorter  wave-lengths. 


26 

Ferric  nitrate  with  sodium  nitrate,  plate  31-b.  Temperatures 
were  24.0°  and  61.6°  C.  The  increase  in  absorption  is  very  great, 
being  about  90%  at  wave-length  460. 

Ferric  nitrate  with  ammonium  nitrate,  plate  3 1  -c.  The  tempera- 
tures were  23.2°  and  57.3°  C.  The  increase  in  absorption  is  very 
great. 

Ferric  nitrate  with  calcium  nitrate,  plate  32-a.  Temperatures 
were  27.3°  and  59.3°  C.  There  is  very  great  increase  in  absorption 
in  the  region  of  the  shorter  wave-lengths. 

Ferric  nitrate  with  aluminium  nitrate,  plate  32-b.  Temperatures 
were  23.5°  and  57.7°  C.  The  curves  show  greatly  increased  absorp- 
tion for  the  shorter  wave-lengths,  the  increase  at  440  being  60%. 

Copper  nitrate.  The  temperatures  were  23.5°  and  70.0°  C. 
No  change  in  absorption  could  be  detected. 

Copper  nitrate  with  aluminium  nitrate.  Up  to  70°  C  no  change 
in  absorption  could  be  detected. 

Copper  nitrate  with  ammonium  nitrate,  plate  34-b.      Tempera 
tures  were  23.5°  and  69.0°  C.     There  is  very  little  if  any  change  in 
absorption. 

Copper  nitrate  with  sodium  nitrate,  plate  34-c.  Temperatures 
were  23.5°  and  66.3 °C.  There  is  considerable  increase  in  absorp- 
tion for  the  region  of  the  longer  wave-lengths. 

Cobalt  nitrate.     Up  to  71 .°  C  the  absorption  seemed  to  be  the  same 
as  at  room  temperature. 

Cobalt  nitrate  with  sodium  nitrate,  plate  30-b.  The  tempera- 
tures were  23.5°  and  74.4°  C.  There  is  very  great  increase  in  ab- 
sorption throughout  the  visible  spectrum,  although  neither  cobalt  ni- 
thate  nor  sodium  nitrate  show  any  temperature  effect. 

Cobalt  sulphate,  plate  30-c.  The  temperatures  were  23.5°, 
47.0°,  and  70.5°  C.  As  the  temperature  rises  there  is  a  steady  in- 
crease in  absorption  throughout  the  spectrum. 

Copper  sulphate,  plate  35-a.  Temperatures  were  23.5°  and 
69.0°  C.  There  is  very  great  increase  in  absorption  in  the  red. 

Copper  sulphate  with  potassium  sulphate,  plate  35-b.  The  tem- 
peratures were  23.5°  and  71.0°  C.  There  is  increased  transparency 
at  about  wave-length  580  and  increased  absorption  in  the  red.  While 
taking  observations  on  this  solution  it  was  discovered  that  the  addition 
of  sulphuric  acid  completely  destroyed  the  temperature  effect. 


27 

ABSORPTION  OF  LIGHT  BY  ALCOHOUC  SOLUTIONS  OF 
ORGANIC  COMPOUNDS 

The  absorption  of  three  compounds:  azobenzene,  benzene  azo 
ortho  cresol  acetate,  and  benzene  azo  phenol  benzoate  was  measured.* 
These  compounds  are  insoluble  in  water,  but  soluble  in  most  organic 
solvents.  Absolute  alcohol  was  used  as  solvent.  This  introduced  a 
new  difficulty,  for  the  alcohol  attacked  the  wax  with  which  the  cells 
were  lined.  Fortunately  a  glass  cell  was  found  that  answered  the 
purpose  very  well.  It  was  thought,  however,  that  this  cell  would  not 
stand  a  very  high  temperature,  so  no  attempt  was  made  to  measure 
the  temperature  effect. 

Azobenzene,  plate  37-a.  The  concentration  was  .01841.  At 
this  concentration  the  solution  is  a  deep  red.  The  solution  is  practi- 
cally opaque  between  wave-lengths  440  and  480.  From  570  to  the 
red  end  of  the  spectrum  it  is  quite  transparent.  The  equation  of  this 
curve  was  determined  as  shown  in  the  graph.  Unlike  all  the  other 
curves  whose  equations  have  been  determined  it  is  not  exponential  but 
a  power  curve. 

Benzene  azo  ortho  cresol  acetate,  plate  37-b.  The  concentra- 
tion was  .005961 .  At  this  concentration  the  solution  is  a  beautiful  red. 
From  the  curve  it  will  be  noticed  that  there  is  very  great  absorption 
in  the  violet  and  blue.  From  wave-length  560  to  the  red  end  of  the 
spectrum  it  is  quite  transparents. 

Benzene  azo  phenol  benzoate,  plate  37-c.  The  concentration  was 
.009536.  The  color  of  the  solution  is  that  of  a  beautiful  red.  The 
absorption  curve  is  almost  exactly  the  type  of  curve  found  for  the 
preceding  compound,  but  the  extinction  coefficient  is  not  so  great. 
There  is,  as  before,  very  great  absorption  in  the  violet.  From  wave- 
length 540  to  720  it  is  almost  entirely  transparent. 

RATIO  OF  ABSORPTION  OF  MIXTURE  TO  SUM  OF 
ABSORPTIONS  OF  COMPONENT  PARTS. 

A  critical  study  of  the  various  cases  discussed  on  the  preceding 
pages  and  shown  graphically  on  the  sheets  that  follow,  brings  to  light 
a  very  curious  fact.  The  ratio  of  the  absorption  of  the  mixture  to 
the  sum  of  the  absorptions  of  the  component  parts  has  a  minimum 

"Kindly  furnished  by  Doctor  Boord,  of  the  Chemistry  Department. 


28 

somewhere  between  wave-lengths  510  and  610  milli-microns,  the 
average  position  of  this  minimum  being  about  580.  Or,  expressed 
another  way,  if  the  ordinate  of  the  full  line  curve  is  divided  by  the  or- 
dinate  of  the  dotted  curve,  and  if  the  ratio  thus  determined  is  plotted 
as  a  function  of  the  wave-length,  the  resulting  curve  shows  a  minimum 
near  the  middle  of  the  visible  spectrum.  Some  of  the  most  striking 
examples  are  shown  on  plate  38.  This  tendency  is  more  or  less  ap- 
parent in  about  60  out  of  a  total  of  1 03  sets  of  curves  taken  at  iroom 
temperature,  so  it  cannot  be  attributed  to  coincidence.  Furthermore, 
it  is  believed  that  this  tendency  would  manifest  itself  in  a  greater  num- 
ber of  cass  were  it  not  for  the  fact  that  a  great  many  solutions  are  so 
nearly  transparent  for  one  or  the  other  half  of  the  spectrum  that  no 
accurate  measurements  are  possible  over  that  region. 

On  first  thought  it  seems  rather  remarkable  that  the  result  of 
any  kind  of  mixing  of  salts  should  be  to  increase  the  absorption  at 
the  ends  of  the  spectrum  at  the  expense  of  the  middle.  After  a  little 
[reflection,  however,  a  possible  explanation  suggsts  itslf.  All  of  the 
colored  salts  studied  in  these  series  of  experiments,  with  the  exception 
of  the  cobalt  and  chromium  salts,  have  their  main  absorption  bands  (in 
so  far  as  the  visible  spectrum  is  concerned)  at  one  or  the  other  end  of 
the  spectrum.  Moreover,  the  cobalt  salts  have  their  maximum  ab- 
sorption to  the  left  of  this  minimum  region  under  discussion,  and 
chromium  nitrate  has  its  principle  absorption  in  the  violet  and  blue. 
Besides,  chromium  chloride  shows  but  little  change  in  absorption  due 
to  the  addition  of  colorless  salts,  as  compared  with  similar  changes 
in  most  of  the  other  colored  salts  under  investigation.  Eliminating 
chromium  chloride,  then,  from  the  present  discussion,  we  see  that 
most  of  the  absorpiton  comes  near  the  ends  of  the  spectrum.  Now 
if  we  may  assume  that  the  main  absorption  bands  are  due  to  the 
molecules  and  not  to  the  ions,  and  if  the  mixing  of  salts  tends  to  drive 
the  ions  back  into  the  molecular  state,  we  should  expect  to  increase 
the  absorption  in  the  region  of  the  main  absorption  bands  and  de- 
crease it  elsewhere.  And  this  is  just  what  happens  in  the  large  ma- 
jority of  cases.  The  most  striking  examples  are  shown  on  plate  8. 

EQUATIONS  OF  CURVES. 

The  equations  of  a  number  of  curves  were  determined,  and  with 
one  exception,  found  to  be  simple  exponential  equations.  For  some 
of  these  curves  the  points  computed  from  the  equation  fall  directly  on 


29 

top  of  the  experimental  curve.  In  all  cases  the  greatest  deviations 
occur  in  those  regions  where  the  absorption  is  slight  and  the  conse- 
quent experimental  error  very  large.  This  remairkably  close  agree- 
ment between  mathematical  and  experimental  curves  is  quite  surpris- 
ing for  two  reasons :  first,  that  the  photometric  readings  had  actually 
been  taken  with  sufficient  accuracy  to  give  such  regular  curves ;  and, 
second,  that  the  absorption  should  obey  a  simple  exponential  law  so 
closely.  The  interpretation  of  this  fact  is  not  at  present  clear  to  the 
writer,  but  he  hopes  to  be  able  to  make  some  report  on  the  subject 
later. 

ANOMALOUS  TEMPERATURE  EFFECTS. 

Neither  cobalt  nitrate  nor  sodium  nitrate  shows  any  appreciable 
change  in  absorption  with  increase  of  temperature,  yet  the  mixed  so- 
lution shows  an  exceedingly  large  temperature  effect.  The  same 
thing  is  true  of  nickel  chloride  and  sodium  chloride,  of  chromium 
chloride  and  potassium  chloride,  and  of  cobalt  sulphate  and  potassium 
sulphate.  These  examples  suggest  that  the  colorless  salt  plays  the 
part  of  a  catalytic  agent  in  accelerating  secondary  chemical  reactions, 
or  in  accelerating  or  retarding  the  dissocciation  of  the  colored  salt. 
It  was  also  found  that  the  addition  of  sulphuric  acid  to  the  mixture  of 
copper  sulphate  and  potassium  sulphate  completely  destroyed  the 
temperature  effect.  It  would  seem  that  this  could  be  easily  explained 
on  the  assumption  that  the  excess  of  the  hydrogen  ions  from  the  dis- 
sociated sulphuric  acid  prevented  the  OH  radicals  from  the  dissoci- 
ated water  from  uniting  with  the  other  salts  with  the  formation  of  col- 
ored hydroxides. 

CONCLUSIONS. 

( 1 )  These  experiments  show  that  the  change  in  absorption  pro- 
duced by  the  mixing  of  sulphates  with  other  sulphates  seem  to  be  small 
in  comparison  with  corresponding  changes  in  nitrates,  and  still  smaller 
in  comparison  with  the  changes  obtained  in  the  mixtures  of  chlorides. 

(2)  The  influence  of  temperature  on  the  absorption  of  solutions 
of  mixed  salts  seem  to  be  greatest  in  chlorides.     From  this  fact,  to- 
gether with  the  preceding,  it  seems  that  Ostwald's  idea  that  the  col- 
ored ion  is  alone  effective  in  the  absorption  of  light  if  not  well  founded, 


30 

as  Jones  has  already  pointed  out.  For  evidently  the  radical  has  con- 
siderable influence. 

(3  When  the  ratio  of  the  absorption  of  the  mixture  to  the  sum  of 
the  absorptions  of  the  component  parts  is  plotted  as  a  function  of  the 
wave-length,  the  resulting  curve  generally  shows  a  minimum  near  the 
middle  of  the  spectrum.  An  attempt  has  been  made  to  explain  this 
on  the  two  following  assumptions :  first,  that  the  principal  absorption 
bands  are  due  to  the  molecules  of  the  salt  and  not  the  ions ;  and  sec- 
ond, that  the  effect  of  mixing  different  salts  which  contain  a  common 
radical  is  to  drive  some  of  the  ions  back  into  the  molecular  state,  thus 
increasing  the  concentration  of  the  molecules  of  the  salt  in  the  solu- 
tion. This  would  naturally  increase  the  intensity  of  the  principal 
bands  and  decrease  the  absorption  elsewhere. 

(4)  Where  the  increase  in  absorption  is  excessively  large  and  is 
general  throughout  the  spectrum,  it  is  believed  that  part  of  this  in- 
crease is  due  to  secondary  chemical  reactions.     The  colorless  salt  in 
that  case  probably  plays  the  part  of  a  catalytic  agent  in  accelerating 
these  secondary  reactions. 

(5)  Equations  have  been  determined  for  a  good  many  of  the 
curves  and  found  to  be  exponential  in  form. 

(6)  It  is  believed  that  the  application  of  Jones's  "Solvate  Theory" 
to  the  effects  (recorded  in  this  paper  is  undesirable  and  unnecessary. 
The  absorption  of  light  by  any  colored  electrolyte  is  undoubtedly  a 
function  of  its  dissociation,  and  there  seems  to  be  little  reason  to  doubt 
that  the  degree  of  dissociation  is  changed  upon  the  addition  of  another 
salt  containing  one  radical  common  to  that  of  the  salt  in  solution. 

(7)  No  attempt  has  been  made  to  differentiate  experimentally 
between  scattering,  reflection,  and  true  absorption.     In  the  changes 
recorded  on  the  previous  pages  and  shown  graphically  by  the  curves 
that  follow,  all  three  of  these  phenomena  undoubtedly  play  their  part. 

In  conclusion  I  wish  to  express  my  thanks  to  Profsesor  Alpheus 
W.  Smith,  at  whose  suggestion  the  above  problem  was  undertaken, 
and  whose  helpful  suggestion  and  kindly  council  has  been  a  constant 
inspiration  throughout  the  investigation. 
Physics  Research  Laboratory, 

Ohio  State  University. 


T  A  B  L  B      I. 


COO  P  P  *  R     CHLORIDE. 


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TABLE     II. 


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AUTOBIOGRAPHY. 

I,  Enoch  Franklin  George,  was  born  near  Petersburg,  West  Vir- 
ginia, March  25,  1878.  I  received  practically  all  of  my  secondary 
education  at  West  Virginia  Wesleyan  College,  and  my  undergraduate 
education  at  Valparaiso  University,  Chicago  University,  and  West  Vir- 
ginia University.  From  Valparaiso  University  I  obtained  the  Degree 
of  Bachelor  of  Science  in  1 907,  and  from  West  Virginia  University  the 
Degree  of  Bachelor  of  Arts  in  1914  and  the  Degree  of  Master  of  Arts 
in  1916.  While  in  residence  at  West  Virginia  University  I  acted  in 
the  capacity  of  Assistant  to  Dr.  C.  W.  Waggoner  during  the  year 
1914-1915.  The  summer  of  19141  spent  as  graduate  student  at  the 
University  of  Illinois.  The  year  1915-1916  I  held  a  graduate  as- 
sistantship,  and  the  years  1916-1917  and  1919-1920  a  University 
fellowship  at  the  Ohio  State  University,  from  which  school  I  received 
the  Degree  of  Doctor  of  Philosophy  in  1 920. 


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